1. Field of the Invention
The present invention relates to an insulating device, and more particularly, to an insulating device for a system on chip (SOC) with variable power source.
2. Description of the Related Art
In system on chip (SOC) designs, different operating modes are provided for efficient power utilization, for example, normal operating mode, power-down mode and the like. Thus, a power supply system provided in the SOC design, for example, can include a main source, a battery source and the like. FIG. 1a shows a conventional SOC design with variable power source. The first circuit region 1 is powered by a main source and the second circuit region 2 by a battery source. Output signals of the first circuit region 1 are connected to the second circuit region 2 directly through terminal A. In power-down mode, the main source is turned off and terminal A floated. This may cause current leakage in the second circuit 2 powered by the battery source, reducing the power utilization of the entire system, as shown in FIG. 1b. 
To address the above problem, an insulator for a power supply system with variable source is disclosed by Yu et al. in U.S. Pat. No. 6,593,775. In Yu, a signal isolation controller is used to change an output level of the circuit powered by VCC according to a detected level as shown in FIG. 2. As disclosed in Yu, the signal isolation controller can includes a switch 62 and a tri-state device 61, such as a tri-state buffer, a transmission gate or a latch. In SOC designs, however, utilization of tri-state devices is not conducive to timing analysis.